Directional acoustic signal transducer



Oct. 14, 1958 G. KURTzE ETAL 2,856,022

' DIRECTIONAL AcoUsTIc SIGNAL TRANSDUCER Filed Aug. 6, 1954 2 Sheets-Sheet 1 @fkk W z'a a Oct. 14, 1958 G. KURTzE ET A1. 2,856,022

DIRECTIONAL ACOUSTIC SIGNAL TRSDUCER Filed Aug.: e, 1954 2 Sheets-Sheet 2 assignors to ElectrofSonic,Laboratories, Inc., Long Islandity- N: Y a corporation 0f New York 1A pplicatioti August 6, 1954,` Serial No, 448,332

G Qlams. (Cl. ll)

'J"This invention relates to-y acoustic signal ltrarislur;ers

United States Patent fw suchas microphones and more particularly to directional acoustic signal transducers.

"U: S. Mason" Patent No. 2,225,3l2'discloses, nn eiective. directionally Selective. or. `discriin.inatingsain1d;frnI1S .ducing device wherein thej directi onal discrimination is obtained by an .acoustic impe dance `formed of a large number of adjacent sound transmittingftnbes or ducts extending parallel to va common. clnstenaxis, the. dilerem ducts varyingina predetermined progressive r ela tion in `both length andalsoin theilfQross-sectional areas so that 'the sensitivity of this acoustic impedance is., a maximum for sound waves reachingorincident uponrtheuductsnin directions parallel to the cluster axis. T heV4 endsmof vthe 'ducts nerestthe microphone 0r.transmitter.,arcplanar and equally spaced frointhe diaphragm or otheryibratorf/ ,elements of the transrnittenl fflhe diierentr ducts are of increasing length vsothatthelonsft.ducts has alensth substantially equal to the `longest wavelength orlowest frequency and the shortest ducthasadengthqlalto the shortestwavelength or the lowest frequencyof the A"-lhe' foregoing and other objectsv of the inventionwill be best understood from thefollowing-description of yari- 'o us eXempli-lcations of the features of t he invention, refjerence being had tothe accompanying drawings, wherein:

' Fig. l is a vertical cross-sectional and partially diagrammatic `View of ay directional microphone exemplifying one form of the invention; v Fig. 2 is a cross-sectionaltsviewalong lines 2-2 of Fie- 1; 1

' Fig. 3 is anelevational viewo f a-portion of the directionalduct of'Figs. l-l and 2 showing how vthe damping strip is applied to the duct in accordance with theprinciples exemplifying one form of theinvention; f Fig.'4 is al curve diagram'explanatory ofthe relation- I'shipof thefthickness of applied damping material to the =directional1 discrimination of the: duct forming-part `of a :transducer exemplifying the invention.

fFig. Sis a polar-diagramplot of thefdirectional sound -discrimination obtained with-anelectrostaticmicrophone 2,856,022 Patented Oct., 1 4, l 958 v transducer isachieved with only a single long duct structure having an elongated acoustically pervious region'of successivelyidiierentacoustic perviousness. 'The sound transducer Velementiis connected to one end ofthe duct.

`The acoustic ,perviousness of diilerentlongitudinal porto the transducerelement for causing all sound waves reaching the transducer element from different longitudinal portions ofI the duct to A,undergo rsubstantially the same degree of attenuation irrespective of the length of. the duct paths through which they travel. In other.words,'the diferentlongitudinal portions ofthe longitudinal sound pervious -duct region havesuch different graduated degrees of predetermined sound perviousness as to cause 'all sound waves whichreach the sound transducer topndergothe same degree of attenuation irrespective of the length of the` duct paths along which they travel tothentransducer element.

The directional discrimination achieved by giving such graduated sound perviousness to different longitudinal portions of the duct varies. withthefrequency in that the discrimination increasesin sharpness the greater the length L of the duct in relation, tothe wave length A ofthe with increasing frequency so that the relation oftheelective ductlengthL to the wavelength A of the sound waves or L/ A remains vsubstantiallyconstant kover a vcertain frequency range.

The ductrnay be provided with the desired predetermined different acoustic perviousness in a variety of ways, as by providing it with a series of openings or'pores-of diierent size and length, with or without dilerent'wall thicknesses along -its diierent longitudinal portions, and/or bycombining such openings or, pores with llongitudinally graduated or ungraduated damping means. A simple directionally discriminating acoustic impedance structure is obtained by forming the duct with a slit the width and/or depth of which varies along its length `to provide it with the desired ditferential acoustic perviousness or by combining such duct slit with a damping means which provides fory the desired predetermined different acousticperviousness along different portions of the slit length.

According t-o the invention, a highlyfeffective directionally selective impedance Iis obtainedt gwith an- 4extremely simplestructure by providingy an lelongated 'duct with a ,long narrow vlongitudinal slit and spirally windingina graduated way aroundgthelductand over the ductslit a ribbon of damping-material southatfsuccessive longitudinalportions of the l duct slitghave thereon vsuccessively larger numbersofwindingturnsgof theftdamping ribbon giving a continuously graduated increase in theacoustic l damping to successive longitudinalportionsof thefduct slit.

As stated above,v the directional discrimination of: the

longitudinal,y duct region rhavingygradually varying perviouslnessyto provide the. same attenuation foralL-sound waves reaching the transducer from diierent longitudinal increase ofthe frequency of the sound waves, this being achieved by choosing and dimensioning the damping so as to give a constant value for the ratio of the duct length L to the wave length )t or L/ A for the entire length of the duct slit.

According to a further phase of 'the invention, an acoustic transducer with inverse directional characteristics, i.e. a transducer which discriminates against reception of sound waves from one predetermined direction only, is obtained by coupling a directionally discriminating impedence of the `type described above with an impedance having uniform sensitivity to all directions, both being suitably coupled to the sound transducer such as the microphone.

Figs. l to 3 illustrate one form of a practical directional microphone exemplifying the invention. It coruprises an electro-acoustic transducing device or element such as the microphone 7 which may be ofthe electrostatic, electro-dynamic or any other type, having a vibratory member 8 and a coupler member 9, shown in the form of a hollow duct, by means of which the transducer device 7 is coupled to a directionally discriminating impedance generally designated 10.

The directionally discriminating impedance 10 comprises an elongated duct or duct structure 11 of substantial length corresponding to the Wave length at the lowest frequency for which directivity is wanted.

The duct 11 may be of metal and its two opposite ends closed by or have connected thereto a matching impedance. In the form shown, one end portion of the duct structure 11 is closed by or has connected thereto the microphone device 7. Tthe other or remote end portion of the duct 11 is closed by a cover 12, of metal, for instance, having at its inner side a body portion of acoustic damping material 13 such as a cotton for absorbing sound energy reaching the remote end region of the duct 11.

The duct 11 is provided with an elongated sound-pervious duct region in the form of slit 15 extending substantially throughout the entire length of the duct, the slit 15 providing acoustic coupling between the exterior space and the interior duct space 16 for causing sound from the exterior space to be propagated by way of the duct 15 into the interior duct space and therethrough to the microphone 7 and excite it to produce a corresponding electric output.

In the form shown, the elongated duct 11 has a duct slit 15 of uniform width 17 extending substantially over its entire length between its outer end 18 and its inner end 19 which is nearest to the microphone 7. In order to give `the duct slit the desired graduated acoustic perviousness` which decreases from the outer slit end`18 towards the inner slit end 19, the slit is combined with correspondingly graduated damping means which causes all sound waves reaching the transducer microphone 7 from different longitudinal portions of the duct slit 15 to undergo the same degree of attenuation irrespective of length of the duct paths through which they travel. In other words, the different longitudinal portions of the duct slit 15 are providedwith such graduated amounts of acoustic damping means as to cause all sound waves which reach the sound transducer 7 through different portions of the duct slit 15 to undergo the same degree 4 of attenuation irrespective of the length of the duct path along which they travel to the transducer element.

According to the invenftion, the desired continuously graduated damping for the duct slit 15 of uniform width 17 is provided ,by spirally winding around the duct a ribbon Z1 of damping material or damping element such as woven fabric, so that the spirally wound layers increase in number in a substantially uniform graduated manner from the outer slit end 18 to inner slit end 19 and provide gradually increasing attenuation for the duct slit portions nearer to the microphone 7.

In order to make the directional discrimination independent of the wave frequencies, either the damping material of the ribbon 21 or additional material within the duct is so chosen that the damping effect automatically compensates for any frequency discrimination in that the damping material or the damping elements thereof provide a frequency discrimination which results substantially in or approaches a constant ratio of the effective duct length L to the wave length lambda for the transmitted sound waves or a constant ratio for L/larnbda.

Without in any way thereby limiting the scope of the invention, but in order to enable more ready practice of the same, there are given below specific data of a practical construction of a microphone of the type shown in Figs. l and 2 which has extremely desirable qualities.

The duct 11 is formed of a tube of metal one meter long having a slit 15 about 95 cm.,1ong and about 1 millimeter in width. The specilic duct 11 shown was madeof aluminum tube with a circular cross-section 1 centimeter (cm.) in diameter, 1.5 millimeter (mm.) thick and 1 meter (m.) long. The longitudinal duct slit 15 was about 97 cm. long and about 1 mm. wide.

The damping was provided by a woven ribbon of cotton threads about 22 mm. wide spirally wound around the tube 11 with the number of overlapping spiral ribbon turns increasing toward the microphone 7, and giving the frequency compensated attenuation increasing gradually from the outer slit end 18 to the inner slit end 19 as explained above. In the practical construction of the microphone, good results have been obtained by placing a continuous length of a cotton ribbon 22 over longitudinal portions of the slit 15 before winding around the tube the spiral ribbon turns 21.

In Fig. 4, the ordinates of curve C represent the values of the damping resistance of the duct slit 15 for successive elements of the duct slit, the damping resistance being proportional to the square root of (L-x) where L represents the total length of the slit and x the increments of the slit from the outer slit end 18 in the direction toward the microphone. The curve C represents the desired damping to secure maximum direc tional discrimination. In Fig. 4, the step-like curve elements C1,`C2, C-a` show the damping `secured by successively larger number of spiral turns or layers of a cotton damping ribbon placed upon the slit indicating that such damping material approaches in its damping action the desired `clamping action. Commercially available cotton fabric ribbons may be readily used to provide frequency compensation and graduated damping required to secure the desired large directional discrimination.

Fig. 5 is a polar diagram plot of the directional sound discrimination obtained with an electrostatic microphone provided with the specific directionally discriminating impedance duct shown and described above in connection with Figs. 1-3 for sound waves of different frequencies.

In the form shown in Figs. l to 3, the duct 11 with the damping ribbon wound thereon may be enclosed in `an outer enclosure 21 of plastic material having a wide elongated gap 32 through which the duct slit15 is exposed to sound waves in the surrounding space. A protective screen 33 of wire for instance is placed along the enclosure gap 32 to protect the underlying damping material 21.

In the polar diagram of Fig. 5, the duct 11 is indicated at the center or Zero point of the diagram with the slit 15 dening the discrimination axis along which sound reaching the duct 11 will be propagated by the duct 11 with the greatest sensitivity.

The features and principles underlying the invention described above in connection with specitic exemplications, will suggest to those skilled in the art many other modifications thereof. It is accordingly udesired that the appended claims be construed broadly and that they shall not be limited to the specific details shown and described in connection with exemplications thereof.

We claim:

1. In a signal transducer for directionally discriminating in the transmission of acoustic signals between an electro-acoustic transducer element and exterior space, an elongated axially directed duct structure confining an elongated interior space having a transducer end portion coupled to said transducer element for generating therein signals in response to sound Waves transmitted in said interior space, said duct structure having a sound-absorbing closure body closing a remote end portion thereof, said duct structure having a substantially continuous elongated sound-pervious duct region extending from near the transducer end portion to near the remo-te end portion of said duct structure for coupling said interior duct space kto the exterior space and causing sound waves from the exterior space to be propagated through said interior space to said transducer element, the length of said pervious duct region being at least ten times greater than the maximum width of said duct and corresponding in length to the lowest frequency for which directional discrimination is eifective, successive longitudinal elements of said pervious duct region having attenuation means of successively diiferent attenuation characteristics which increase in the direction toward said transducer end portion for gradually increasing attenuation of sound waves transmitted from the exterior space through portions of said sound-pervious region which are successively nearer to said transducer element, and thereby causing sound waves transmitted from said exterior space to said transducer element by way of said interior space to undergo substantially the same degree of attenuation irrespective of the length of said interior space through which said sound waves pass before reaching said transducer element.

2. In a signal transducer for directionally discriminating in the transmission of acoustic signals between an electro-acoustic transducer element and exterior space, an elongated axially directed duct structure conning an elongated interior space having a transducer end portion coupled to said transducer element for generating therein signals in response to soundwaves transmitted to said interior space, said duct structure having a sound-absorbing closure body closing a remote end portion of said duct, said duct structure having a substantially continuous elongated slit-like opening extend-ing from near the transducer end portion to near the remote end portion of said duct structure for coupling said interior duct space to the exterior space and causing sound waves from the exterior space to be propagated through said interior space to said transducer element, the length of said slit-like opening being at least ten times greater than the maximum width of said duct and corresponding in length to the lowest frequency for which directional discrimination is eective, successive longitudinal elements of said slit-like opening having attenuation means of successively diiferent attenuation characteristics which increase in the direction toward said transducer element for gradually increasing attenuation of sound waves transmitted from the exterior space through portions of said slit-like opening which are successively nearer to said transducer element and thereby causing sound waves transmitted from said exterio-r space to said transducer element by way of said interior space to undergo substantially the same degree of attenuation irrespective of the length of Said interior space through which said sound waves pass before reaching said transducer element.

3. In a signal transducer as claimed in claim 2, said attenuating means including flexible sheet material placed over said slit-like opening.

4. In a signal transducer as claimed in claim 2, said attenuating means including a strip of damping material placed over said slit-like opening.

5. In a signal transducer as claimed in claim 2, said attenuating means including a strip of attenuating material spirally wound over at least a major portion of the length of said slit-like opening. t

6. In a signal transducer as claimed in claim 2, said attenuating means including a strip of damping material spirally wound over at least a major portion of the length of said slit-like opening, said strip being wound with a pitch which is increasingly shorter in the direction toward said transducer end portion.

References Cited in the le of this patent t UNITED STATES PATENTS 

